Over View of Respiration in Plants

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Lee Cain
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1 Ecophysiology Please Note: Some of the slides are Animated and are best viewed as a Slide Show; some slides have attached notes below the slides and these are best viewed in Normal (editing) view.

2 3. Respiration

Pentose Phosphate Pathway Cytoplasm (Oxidative) Plastid

3 Over View of Respiration in Plants There are 3 Key Pathways 1.Glycolysis Cytoplasm Plastids 2.Pentose Phosphate Pathway Cytoplasm (Oxidative) Plastid (Reductive C3 Cycle Running Backwards) 3.Krebs (TCA) Cycle Mitochondria

Reducing Potential for Biosynthesis NADPH from Oxidative PPP, NADH & FADH 2 in TCA Cycle

4 Respiration Generates Metabolites, Reducing Potential, & ATP Role: 1.Precursors for Biosynthesis Pentose sugars, amino acids, phenolic compounds, etc. 2.Reducing Potential for Biosynthesis NADPH from Oxidative PPP, NADH & FADH 2 in TCA Cycle 3. Substrate Level ATP Glycolysis & TCA Cycle 4.Reducing Potential for Chemiosmotic ATP synthesis (Mitochondria)

5 Cy Deatils of Glycolysis Carbon Entry From Hexose Triose Phosphate Hexose 2 ATP Consumed 4 ATP Generated 1 NADH Generated Triose Phosphate 2 ATP Generated 1 NADH Generated

6 Features of Plant Oxidative Phosphorylation Complex I (NADH/UQ Oxidoreductase) 4 H + Pumped Sensitive to Rotenone & Piericidin Complex II (Succinate Dehydrogenase, FADH 2 / UQ Oxidoreductase) Does Not Pump H + Complex III (UQH 2 / Cyctochrome C Oxidoreductase) Pumps 4 H + Sensitive to Antimycin Complex IV (Cyt C / H 2 O Oxidoreductase) Pumps 2 H + Sensitive to CO, Azide, CN Alternative Oxidase (AO) Does Not Pump H + Sensitive to SHAM (salicylhydroxamic acid) Cytoplasmic side, Ca 2+ Dependent NADH and NADPH Dehydrogenase Matrix side NADH and Ca 2+ dependent NADPH Dehydrogenase ATP Synthase F0/F1 Type enzyme, ~ 3H + / ATP Uncoupling Protein (UCP) A Protonophore that Uncouples ATP synthesis from Mitochondrial Electron Transport

7 Use of Photosynthate in Plants Proportions Vary with Species & Environmental Conditions, but... Most Photosynthate is Used for Growth Up to ½Used for Respiration Growth, Maintenance, Transport

8 RQ & P:O Ratios Respiratory Quotient (RQ) Varies CO 2 Released / O 2 Consumed, dependent on P:O ratio oxidation state of substrate lipids ~ 0.4, hexoses ~1.0 Tissues photosynthetic ~ 1.0, N fixing roots ~1.6 ATP Produced / O2 Consumed <1.0 to >2.5

9 RQ Increases with Growth Rate Hexose Respiration for ATP gives RQ ~ 1.0 Photosynthesis Consumes CO 2 so RQ Rises

10 Carbon Enters Mitochondria as Pyruvate, Malate, & Glycine Malate & Pyruvate are Major Metabolites in non Photosynthesising (green and non green) Tissues Glycine (from Photorespiration) is the Main Metabolite in Photosynthesising Green Tissues

11 Products of TCA Cycle Drive Electron Transport

12 Studies on Isolated Intact Mitochondria Reveal ETR is Under Respiratory Control Addition of Uncouplers Show State 2/ State 3 Transition is Controlled Mainly by DH +, not ADP Concentration State 1 Initially Low ETR as no Substrate for TCA Cycle Present State 2 Import of TCA Cycle Intermediates Drive Limited ETRs State 3 Exchange of Matrix Side ATP for Cytoplasmic Side ADP Stimulates ETR State 4 Internal ADP converted to ATP, ETR Stops ETR Under RC i.e. Demand for ATP

13 Chemiosmosis: ATP Production by H + Gradients pmf + RT H = Δμ + = Δϕ ln H + F H out RT pmf = Δμ + = Δϕ ΔpH H F in R = Gas Constant (8.32 J mol 1 K 1 ) T = Temperature K F = Faraday s Constant (96,500 J K.mol mv 1 ) Dj = Membrane Potential (mv) pmf= Proton Motive Force DmH + = electrochemical potential for H + The cytoplasmic side of the mitochondria is well buffered at ~ph 7.5 so the Dj provides most of the driving force for H + movement into the matrix through the ATP Synthase.

14 Major Control Points of Respiration in Plants Hexose Concentration ATP Demand Accumulation of TCA Cycle Intermediates Activation of Complex II Reduction of UQ AO Activation

15 NMR Spectroscopy can be used to Show ATP Synthesis / Hydrolysis In Vivo Peak 4 is g ATP Phosphate Atom Peak 2 is free Phosphate in Cytoplasm Spectrum A: after microwave saturation in g ATP band Spectrum B: control A B: difference Spectrum showing spin polarized P atom appearing in Cytosolic fraction

16 ADP:O Ratios can be Calculated from Concomitant NMR and O 2 Uptake Measurements on Intact Tissues

17 The Rate of ET Through the Cytochome & AO Pathways is Regulated ETR Through Cyt Pathway (V cyt ) is Triggered By Low Reduction State of UQ Pool (Qr/Qt) ETR Through AO Pathway (V alt ) is Triggered by High Reduction State of UQ Pool (>40%) Addition of TCA Cycle Organic Acids Converts AO to Form Activated by Low UQ Pool Reduction State Electron Flow through two Pathways is Regulated by Redox State of UQ Pool AND Concentration of TCA Cycle Intermediates

18 What Role Does the AO Pathway Play? Thermogenesis: Rise of >15C ambient in some specialized tissues e.g. Spadex of Sacred Lotus (Nelumbo nucifera) Skunk Cabbage (Simplocarpus foeitidus) Heart leaf Philodendron (Philodendron selloum ) Seymour & Schultz Motel 1996 Nature 238: 305

19 Thermogenesis Appears to be a Reproductive Strategy Philodendron selloum Simplocarpus foetidis There is no evidence that Thermogenesis can alleviate cold stress in plants

20 What Role Does the AO Pathway Play? Energy Overflow When UQ is Highly Reduced, ROS can be Generated. Activation of the AO when TCA Cycle Intermediates are High Will Decrease the Redox State of UQ and Prevent ROS Generation Uncouples ATP Synthesis from C Skeleton Demand When C Skeleton Demand is High (e.g. Citrate excretion) ATP Requirement is Low Uncouples ATP Synthesis Decreases Redox State of Chloroplast SHAM and KCN decrease Photosynthetic O 2 Evolution. AO may oxidise NAD(P)H Excreted from the Chloroplast in High Light to Prevent over reduction. Allow Limited Oxidative Phosphorylation When Cyt Pathway is Inhibited Plants produce inhibitors of Cyt Pathway (CN, sulphide, CO 2,); AO may provide limited ATP synthesis under these conditions. High Rates of ROS Production Induce Transcription of AO Genes Role in prevention of ROS generation

21 Environmental Effects on Respiratory Processes Flooding, Hypoxia, & Anoxia Diffusion of Gasses in Liquid ~ 10,000 Slower than in Air O 2 Concentration is water is ~40x Lower than in Air Leads to Anaerobic Respiration in Roots of Plants in Flooded / Clay Impacted soils Low ATP Transcriptional Changes e.g. ADH Fermentation Accumulation of Lactate Acidosis inhibits Lactate Dehydrogenase Ethanol Accumulation Inhibits normal PDC Acetyl Co A Activity Accumulation of Acetaldehyde (Very Toxic) Ethanol is Toxic to Humans but not to Plants!

22 Strategies for Avoiding Hypoxia Aerenchyma Pneumatophores (in Mangrove) Lenticels The above rely on Increasing O 2 Diffusion to Roots Through Structural Modifications (& Accelerated Methane Loss to Atmosphere?) May be a Role for Pressure Driven O 2 Flow from Shoot to Root?

23 Respiration Is Affected By Stress & RGR Salinity & Drought Stress Cause an Initial Rapid Increase in Respiration In Soybean seedlings Drought causes rapid decrease in CO 2 Assimilation but no Change in Respiratory O 2 Uptake BUT an Increase in AO pathway ETR Occurs In longer term Compatible Solutes are Synthesized; Energy required for this Matches the Observed Decline in Respiration. Manipulation of N supply shows Growth Rates Correlate well with Respiration

Respiration Is Affected By Stress & RGR Irradiance Transfer to Low Light Decreased Respiration Addition of Sucrose NO INCREASE in Respiration due to REDUCED DEMAND for Energy NOT REDUCED SUPPLY of

24 Respiration Is Affected By Stress & RGR Irradiance Transfer to Low Light Decreased Respiration Addition of Sucrose NO INCREASE in Respiration due to REDUCED DEMAND for Energy NOT REDUCED SUPPLY of Photosynthate Shade Plants Show Low AO Activity (Alocaia) Sun Plants Show High AO Activity (Spinacia) AO Involved in Excess Energy Dissipation? AO Activity is Controlled by Redox Status of Mitochondria Active AO is Monomer, Inactive AO is a Dimer

25 Respiration and Photosynthesis Rates Are Co Regulated by Irradiance

26 Temperature Effects on Respiration Respiration Increases With Temperature Q10 Response Greatest in Low Temperature Acclimated Plants Regardless of Their Origin Plants Adjust Respiration Rates to Achieve Equivalent Rates Regardless of Growth Temperature Respiration is Thermodynamically Limited at Low Temperatures Plants respond by Increasing the Abundance of Respiratory Proteins Subsequent Transfer to Warm Conditions Shows Large Increase in respiration (Q10 >2.5)

27 Respiration Increases With Rhizosphere Acidity Zea mays Seedlings ph 7.0 ph 4.0 It is Difficult to Assign this Respiratory Burst to Any Mechanism Although the Plasma Membrane P Type H + pumping ATPase has been implicated

28 Respiration May Increase in Some Species With Increased Atmospheric CO 2 Concentrations

29 Root And Shoot Respiration Changes With Growth Rate

30 Model for Carbon Utilization During Growth Cs represents the fraction of Biomass that is not Recoverable, e.g. Lignocellulose Cr represents the carbon respired for generating the ATP & NAD(P)H Required for biomass production

31 Estimates of the Partitioning of Carbon into the Molecules of Life

32 Costs of Construction of Plant Organs n=123 n=28 n=35 n=31

33 Maintenance Respiration Does Not Change With Plant Age But Growth Respiration Does

Multiple Regression Analyses Reveal 3 Major Respiratory Sinks Growth Respiration (synthesis of new compounds for biomass accumulation) Maintenance

34 Multiple Regression Analyses Reveal 3 Major Respiratory Sinks Growth Respiration (synthesis of new compounds for biomass accumulation) Maintenance Respiration (repair and housekeeping metabolism) (Ion) Transport Respiration (maintaining ion and ph gradients (homeostasis) and nutrient ion acquisition)

35 Comparisons of Respiratory Sinks in Different Plant Species

36 Conclusions There is a Good Correlation Between: Growth Respiration & RGR Transport Respiration & RGR Maintenance Respiration & RGR Model of 3 Major Respiratory Sinks Fits Data Well Partitioning of Respiratory carbon between these sinks varies with species and habitat Plants have more complex respiratory mechanisms than animals (AO, Protein Uncoupler, etc.) but neither the regulation nor the function is fully understood

Cellular Pathways That Harvest Chemical Energy. Cellular Pathways That Harvest Chemical Energy. Cellular Pathways In General

Cellular Pathways That Harvest Chemical Energy A. Obtaining Energy and Electrons from Glucose Lecture Series 12 Cellular Pathways That Harvest Chemical Energy B. An Overview: Releasing Energy from Glucose